Switching power supply apparatuses have been widely used as power supply apparatuses for electronic devices such as consumer electronic appliances. The switching power supply apparatuses are used for the purpose of improving power efficiency with a reduction in power consumption. The switching power supply apparatus uses a switching operation of a switching element to control an output to be supplied to a load. The switching operation is repetitive switching between on and off. The switching element may be an electronic switch, for example. The control of an output indicates, for example, the stabilization of an output.
A conventional switching power supply apparatus will be described with reference to a drawing.
FIG. 4 is a circuit diagram illustrating the configuration of the conventional switching power supply apparatus.
As illustrated in FIG. 4, a conventional switching power supply apparatus 1 includes an input terminal 3a to which the positive side of a DC power supply 2 is connected, an input terminal 3b to which the negative side of the DC power supply 2 is connected, an electronic switch 4a and a diode 5a which are connected in series between the input terminals 3a and 3b, a diode 5b and an electronic switch 4b which are connected in series between the input terminals 3a and 3b, an inductor 6a having one end connected to the connection point between the electronic switch 4a and the diode 5a, an inductor 6b having one end connected to the connection point between the diode 5b and the electronic switch 4b, an output terminal 8a to which the other ends of the inductors 6a and 6b are connected and one end of a load 7 is connected, a diode 5d and an electronic switch 4d which are connected in series between the input terminals 3a and 3b, an electronic switch 4c and a diode 5c which are connected in series between the input terminals 3a and 3b, an inductor 6c having one end connected to the connection point between the electronic switch 4c and the diode 5c, an inductor 6d having one end connected to the connection point between the diode 5d and the electronic switch 4d, an output terminal 8b to which the other ends of the inductors 6c and 6d are connected and the other end of the load 7 is connected, a capacitor 9 connected between the output terminals 8a and 8b, and an electronic switch control circuit 10 for controlling the on/off control terminals of the electronic switches 4a, 4b, 4c and 4d. 
The electronic switch control circuit 10 provides two outputs. One of the outputs controls the on/off control terminals of the electronic switches 4a and 4d and the other output controls the on/off control terminals of the electronic switches 4c and 4b. 
An operation of the conventional switching power supply apparatus 1 configured thus will be described. For example, the case of an AC output with a frequency of 50 Hz being generated from a DC output supplied from the DC power supply 2 will be described.
The electronic switch control circuit 10 generates a square wave with a frequency of 20 kHz. In the first half cycle (10-msec) period of a frequency of 50 Hz, the electronic switches 4a and 4d perform a switching operation at 20 kHz whereas the electronic switches 4c and 4b stay in the off state. Accordingly, a current flows in the direction of arrow 11a. 
In the next half cycle (10-msec) period, the electronic switches 4c and 4b perform a switching operation at 20 kHz and the electronic switches 4a and 4d stay in the off state. Accordingly, a current flows in the direction of arrow 11b. 
In this way, a current flows in the direction of arrow 11a in the first half cycle (10-msec) period and a current flows in the direction of arrow 11b in the next half cycle (10-msec) period. This operation is repeated every half cycle of a frequency of 50 Hz. To be specific, repetitive alternation between the first half cycle period in which a current flows in the direction of arrow 11a and the second half cycle period in which a current flows in the direction of arrow 11b generates an alternating current with a frequency of 50 Hz between the output terminals 8a and 8b. 
The inductors 6a and 6d and the capacitor 9 form a first low-pass filter whereas the inductors 6c and 6b and the capacitor 9 form a second low-pass filter. The first and second low-pass filters attenuate harmonic components caused by the switching operation at a frequency of 20 kHz. As a result, the alternating current generated between the output terminals 8a and 8b becomes a 50-Hz sinusoidal wave which oscillates smoothly.
The inductors 6a, 6b, 6c and 6d limit a surge current generated when the electronic switches 4a and 4d and the electronic switches 4c and 4b turn on and off.
The inductors 6a and 6b are inserted in series in a series circuit made up of the DC power supply 2 and the electronic switches 4a and 4b. The inductors 6c and 6d are inserted in series in a series circuit made up of the DC power supply 2 and electronic switches 4c and 4d. Accordingly, the DC power supply 2 is not shorted even if the electronic switches 4a and 4b or electronic switches 4d and 4c are turned on at the same time when the electronic switches performing a switching operation switch.
Prior art information relating to the present invention is disclosed in Patent Literature 1, for example. From page 3, upper right column, line 2, to page 3, lower left column, line 8, of patent literature 1, there is a description of inserting a coil between switching elements that alternately turn on and off, in order to prevent current and voltage surges caused by simultaneous switching on of the switching elements and also prevent damage to the switching elements due to a short-circuit current.